The present invention relates to an induction motor drive which carries out adjustable speed operation of an induction motor by an inverter and more particularly to technique effective when one inverter fails and the inverter is changed over from the failed inverter to another inverter to drive the induction motor.
The induction motor drive for carrying out adjustable speed operation of the induction motor by the inverter is widely used to carry out adjustable speed operation of various machines (hereinafter referred to as load) such as fan and pump. Further, in case of an important load which is not allowed to be stopped even if failure occurs, inverters are duplexed and when one inverter fails, the inverter is changed over from the failed inverter to the other inverter to continue the adjustable speed operation without stopping operation of the load.
For example, in an induction motor drive described in JP-A-5-336787 (patent document 1), a failure signal of a failed inverter is sent to the other inverter and after the other inverter confirms changeover of a switch connected between the induction motor and the other inverter, the inverter starts the operation of the induction motor. Especially, the other normal inverter sets a starting frequency so as to match the speed of the induction motor rotating inertially upon changeover, so that the changeover is made smoothly while the operation of the inverter is synchronized with the speed of the induction motor. Consequently, even if one inverter fails, it can be changed over to the other inverter, so that the adjustable speed operation of the induction motor can be continued without stopping operation of the induction motor.
Further, JP-A-8-116675 (patent document 2) discloses that only inverter main circuits of an inverter are duplexed and an inverter controller is provided in common to both the inverter main circuits so that the same drive signals are always supplied to both the inverter main circuits and output voltages, phases and frequencies of both the inverter main circuits are synchronized with each other so that the inverter main circuits are changed over from one to the other. Consequently, variation in output (torque shock and the like) of a load upon changeover can be eliminated.
On the other hand, JP-A-5-83989 (patent document 3) discloses a changeover operation apparatus including an adjustable speed inverter which drives induction motors upon sudden adjustable speed operation at the time that induction motors are separated from a spinning line in which induction motors are group-operated and a normal operation inverter which is operated by changing over from the adjustable speed inverter to the normal operation inverter when the induction motors are normally operated in the spinning line. Especially, the inverter to be used after changeover is previously started and a frequency and a phase of an output voltage of the inverter used before changeover are detected, so that the inverters are changed over after the frequency and the phase of the output voltage of the inverter to be used after changeover are synchronized with those of the inverter used before changeover to thereby suppress variation in speed of the induction motor and shorten the changeover time.